Subject: Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap

From: Laurent Dufour <[email-masked]>
Date: Thu, 26 Mar 2015 11:37:33 +0100

On 26/03/2015 10:43, Ingo Molnar wrote:
> 
> * Benjamin Herrenschmidt <[email-masked]> wrote:
> 
>> On Wed, 2015-03-25 at 19:36 +0100, Ingo Molnar wrote:
>>> * Ingo Molnar <[email-masked]> wrote:
>>>
>>>>> +#define __HAVE_ARCH_REMAP
>>>>> +static inline void arch_remap(struct mm_struct *mm,
>>>>> +			      unsigned long old_start, unsigned long old_end,
>>>>> +			      unsigned long new_start, unsigned long new_end)
>>>>> +{
>>>>> +	/*
>>>>> +	 * mremap() doesn't allow moving multiple vmas so we can limit the
>>>>> +	 * check to old_start == vdso_base.
>>>>> +	 */
>>>>> +	if (old_start == mm->context.vdso_base)
>>>>> +		mm->context.vdso_base = new_start;
>>>>> +}
>>>>
>>>> mremap() doesn't allow moving multiple vmas, but it allows the 
>>>> movement of multi-page vmas and it also allows partial mremap()s, 
>>>> where it will split up a vma.
>>>
>>> I.e. mremap() supports the shrinking (and growing) of vmas. In that 
>>> case mremap() will unmap the end of the vma and will shrink the 
>>> remaining vDSO vma.
>>>
>>> Doesn't that result in a non-working vDSO that should zero out 
>>> vdso_base?
>>
>> Right. Now we can't completely prevent the user from shooting itself 
>> in the foot I suppose, though there is a legit usage scenario which 
>> is to move the vDSO around which it would be nice to support. I 
>> think it's reasonable to put the onus on the user here to do the 
>> right thing.
> 
> I argue we should use the right condition to clear vdso_base: if the 
> vDSO gets at least partially unmapped. Otherwise there's little point 
> in the whole patch: either correctly track whether the vDSO is OK, or 
> don't ...

That's a good option, but it may be hard to achieve in the case the vDSO
area has been splitted in multiple pieces.

Not sure there is a right way to handle that, here this is a best
effort, allowing a process to unmap its vDSO and having the sigreturn
call done through the stack area (it has to make it executable).

Anyway I'll dig into that, assuming that the vdso_base pointer should be
clear if a part of the vDSO is moved or unmapped. The patch will be
larger since I'll have to get the vDSO size which is private to the
vdso.c file.

> There's also the question of mprotect(): can users mprotect() the vDSO 
> on PowerPC?

Yes, mprotect() the vDSO is allowed on PowerPC, as it is on x86, and
certainly all the other architectures.
Furthermore, if it is done on a partial part of the vDSO it is splitting
the vma...